Abstract
Cells of the primitive streak in both chick and penguin embryos demonstrate elongate bottle shapes as well as cuboidal-columnar shapes. We suggest that a change in cell shape is a requirement for invagination. The change in shape of primitive streak cells may be initiated by the elaboration of cytoplasmic microtubules; these may act by transporting cytoplasm from apical to basal portions of epiblast cells. Bundles of microtubules are found within and arranged parallel to attenuated necks of invaginating epiblast cells. The mode of action of these microtubules is not known, but within elongate neck regions microtubules are long and straight rather than coiled. Due to cytoplasmic translocation plus the concerted action of lobopodial and filopodial cytoplasmic extensions, and specialized cell contacts, primitive streak cells are displaced beneath the epiblast's free surface into the mesoblast.
Published Version
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